Personal blog of Yzmir Ramirez

If we have InnoDB pages there are two ways to learn how many records they contain:

• PAGE_N_RECS field in the page header
• Count records while walking over the list of records from infimum to supremum

In some previous revision of the recovery tool a short summary was added to a dump which is produced by the constraints_parser.

But if a page is lost and page_parser hasn’t found it, all records from this page are lost. In other words per-page recovery statistics gives us little idea about whether or not a recovered table is complete.

To cover this flaw a new tool index_check is introduced in the revision 80

As you might know InnoDB stores a table in a clustered index called PRIMARY.

The PRIMARY index is a B+tree structure. It has a nice feature that all leaf nodes have pointers to a next leaf node, so following the pointers we can read whole index in the primary key order. InnoDB extends the structure and stores also a pointer to the previous node. The previous node of the head is NULL as well as the next node of the tail.

Based on this knowledge we can check if we have all elements of the list. If we can get from any InnoDB page to the beginning and the end of the list then our set of pages is complete.

This is exactly what index_chk does – it reads files from a directory with InnoDB pages (that is produced by page_parser) and tries to walk over the list of pages back and forth.

Let’s review an example. I took some corrupt InnoDB tablespace and split it with a page_parser:

# ./page_parser -f /var/lib/mysql/ibdata1
Opening file: /var/lib/mysql/ibdata1
File information:
ID of device containing file:        64512
inode number:                     27037954
protection:                         100660 (regular file)
number of hard links:                   1
user ID of owner:                      107
group ID of owner:                     116
device ID (if special file):             0
blocksize for filesystem I/O:         4096
number of blocks allocated:          86016
time of last access:            1374662334 Wed Jul 24 06:38:54 2013
time of last modification:      1374233938 Fri Jul 19 07:38:58 2013
time of last status change:     1374233938 Fri Jul 19 07:38:58 2013
total size, in bytes:             44040192 (42.000 MiB)
Size to process:                  44040192 (42.000 MiB)
8.26% done. 2013-07-24 08:39:58 ETA(in 00:00 hours). Processing speed: 3637248 B/sec
...
95.70% done. 2013-07-24 08:40:09 ETA(in 00:00 hours). Processing speed: 4399053 B/sec
#

Now let’s take some index and check if it has all pages:

# ls pages-1374669586/FIL_PAGE_INDEX/0-410/
00000000-00000145.page  00000000-00000235.page  00000000-00000241.page  00000000-00000247.page  00000000-00000254.page
00000000-00000147.page  00000000-00000236.page  00000000-00000243.page  00000000-00000249.page  00000000-00000255.page
00000000-00000148.page  00000000-00000239.page  00000000-00000244.page  00000000-00000251.page
# ./index_chk -f pages-1374669586/FIL_PAGE_INDEX/0-410
Couldn't open file pages-1374669586/FIL_PAGE_INDEX/0-410/00000000-00000140.page
#

Bad news, a page with id 140 is missing!

Indeed the previous page before page#145 is page#140, but it’s missing.

# hexdump -C pages-1374669586/FIL_PAGE_INDEX/0-410/00000000-00000145.page | head -2
00000000  d4 cd 68 41 00 00 00 91  00 00 00 8c ff ff ff ff  |..hA............|
00000010  00 00 00 00 2b 6c ea 90  45 bf 00 00 00 00 00 00  |....+l..E.......|
#

For a cheerful ending let’s check an index that has all pages:

# ls pages-1374669586/FIL_PAGE_INDEX/0-2/* # this is SYS_COLUMNS table
pages-1374669586/FIL_PAGE_INDEX/0-2/00000000-00000010.page  pages-1374669586/FIL_PAGE_INDEX/0-2/00000000-00002253.page
pages-1374669586/FIL_PAGE_INDEX/0-2/00000000-00002244.page
# ./index_chk -f pages-1374669586/FIL_PAGE_INDEX/0-2
OK
#

Thus, a table is fully recovered if and only if two conditions are met:

• Command grep “Lost records: YES” table.dump | grep -v “Leaf page: NO” outputs nothing
• ./index_chk -f pages-1374669586/FIL_PAGE_INDEX/<inde_id of=”” my=”” table=””> reports OK

The post Checking B+tree leaf nodes list consistency in InnoDB appeared first on MySQL Performance Blog.

MySQL 5.6 introduces a new feature – microseconds resolution in some temporal types. As of 5.6.4 TIME, TIMESTAMP and DATETIME can have a fractional part. To create a field with subseconds you can specify precision in brackets: TIME(3), DATETIME(6) etc.

Obviously, the new feature requires the format change. All three types may now have a tail with a fractional part. The tail may be one, two or three bytes long and can store up to six digits after the point.

The non-fractional part has changed too. Thus, DATETIME uses only five bytes comparing to eight in previous versions.

As of revision 79 the recovery toolkit supports the new format.

Let’s recover a sample table to see how it works. The table is

CREATE TABLE `t` (
  `id` int(11) AUTO_INCREMENT,
  `t1` time(3),
  `t2` timestamp(6),
  `t3` datetime(1),
  `t4` datetime,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB

mysql> select * from t;
+----+--------------+----------------------------+-----------------------+---------------------+
| id | t1           | t2                         | t3                    | t4                  |
+----+--------------+----------------------------+-----------------------+---------------------+
|  1 | 05:05:10.000 | 2013-07-04 05:05:10.000000 | 2013-07-04 05:05:10.0 | 2013-07-04 05:05:10 |
|  2 | 05:14:24.414 | 2013-07-04 05:14:24.125000 | 2013-07-04 05:14:24.4 | 2013-07-04 05:14:25 |
|  3 | 05:14:32.566 | 2013-07-04 05:14:32.207031 | 2013-07-04 05:14:32.3 | 2013-07-04 05:14:32 |
|  4 | 05:14:34.344 | 2013-07-04 05:14:34.507813 | 2013-07-04 05:14:34.5 | 2013-07-04 05:14:35 |
|  5 | 05:14:45.348 | 2013-07-04 05:14:45.832031 | 2013-07-04 05:14:45.1 | 2013-07-04 05:14:45 |
+----+--------------+----------------------------+-----------------------+---------------------+

First, we should gerenrate a table definition file:

./create_defs.pl --db test --table t > include/table_defs.h

Then recompile the tool and split a table space that contains records of the table t:

./page_parser -f /var/lib/mysql/test/t.ibd

The next step is to get records from the InnoDB index:

$ ./constraints_parser -5f pages-1372929630/FIL_PAGE_INDEX/0-30/00000000-00000003.page  2> /dev/null
-- Page id: 3, Format: COMPACT, Records list: Valid, Expected records: (5 5)
0000000007DB    A6000001A20110  t       1       "05:05:10.0"    "2013-07-04 05:05:10.0" "2013-07-04 05:05:10.0" "2013-07-04 05:05:10.0"
0000000007DE    A8000001530110  t       2       "05:14:24.4140" "2013-07-04 05:14:24.125000"    "2013-07-04 05:14:24.40"        "2013-07-04 05:14:25.0"
0000000007EC    B2000001A40110  t       3       "05:14:32.5660" "2013-07-04 05:14:32.207031"    "2013-07-04 05:14:32.30"        "2013-07-04 05:14:32.0"
0000000007ED    B3000001A50110  t       4       "05:14:34.3440" "2013-07-04 05:14:34.507813"    "2013-07-04 05:14:34.50"        "2013-07-04 05:14:35.0"
0000000007FB    BD000001670110  t       5       "05:14:45.3480" "2013-07-04 05:14:45.832031"    "2013-07-04 05:14:45.10"        "2013-07-04 05:14:45.0"
-- Page id: 3, Found records: 5, Lost records: NO, Leaf page: YES

To load this dump use the LOAD DATA INFILE command that’s generated by the contraints_parser, I then redirected it to /dev/null in the example above.

As you can see it’s pretty straightforward. There are two notes:

• Because of format differences the tool can detect whether the field is in the new format or old
• For the TIME field (w/o fractional part) in the new format you need to give a hint to contraints_parser. This is -6 option. Let me illustrate this

Before 5.6.4 TIME was packed in three bytes: DD×24×3600 + HH×3600 + MM×60 + SS. As of 5.6.4 it still uses three bytes, but format is different:

TIME new format 

bits	description	values
1	sign	1= non-negative, 0= negative
1	unused	reserved for future extensions
10	hour	0-838
6	minute	0-59
6	second	0-59

If a field is created without a fractional part it’s impossible to determite the format from a field value. Let’s take a table:

CREATE TABLE `t` (
  `id` int(11) AUTO_INCREMENT,
  `t1` time ,
  `t2` time(3) ,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1

mysql> select * from t;
+----+----------+--------------+
| id | t1       | t2           |
+----+----------+--------------+
|  1 | 11:01:17 | 11:01:17.000 |
|  2 | 11:01:17 | 11:01:17.125 |
|  3 | 11:01:17 | 11:01:17.125 |
+----+----------+--------------+

If there is no fractional part constraints_parser assumes old format. So if we try to recover the the records from the table above the result will be wrong:

$ ./constraints_parser -5f pages-1373023772/FIL_PAGE_INDEX/0-31/00000000-00000003.page
-- Page id: 3, Format: COMPACT, Records list: Valid, Expected records: (3 3)
000000000807    C5000001AC0110  t8      1       "12:32:17"      "11:01:17.0"
000000000807    C5000001AC011C  t8      2       "12:32:17"      "11:01:17.1250"
000000000807    C5000001AC0128  t8      3       "12:32:17"      "11:01:17.1250"
-- Page id: 3, Found records: 3, Lost records: NO, Leaf page: YES

Thus, we need to give a hint, then TIME values are correct:

./constraints_parser -5f pages-1373023772/FIL_PAGE_INDEX/0-31/00000000-00000003.page -6
-- Page id: 3, Format: COMPACT, Records list: Valid, Expected records: (3 3)
000000000807    C5000001AC0110  t8      1       "11:01:17.0"    "11:01:17.0"
000000000807    C5000001AC011C  t8      2       "11:01:17.0"    "11:01:17.1250"
000000000807    C5000001AC0128  t8      3       "11:01:17.0"    "11:01:17.1250"
-- Page id: 3, Found records: 3, Lost records: NO, Leaf page: YES

The post Recovering temporal types in MySQL 5.6: TIME, TIMESTAMP and DATETIME appeared first on MySQL Performance Blog.